Method of chemically treating metals



United States Patent 3,257,295 METHOD OF CHEMICALLY TREATING METALSShigeru Yonezaki, Yawata, Minoru Kamata and Tsuneyasu Watanabe, Tobata,and Kango Sakai and Kazuo Ikegami, Yawata, Japan, assignors to YawataIron & Steel Co., Ltd, Tokyo, Japan, a corporation of Japan No Drawing.Filed Jan. 14, 1963, Ser. No. 251,002 7 Claims priority, applicationJapan, Jan. 20, 1962,

' 37/ 1,867 6 Claims. (Cl. 204-56) This invention relates to methods ofchemically treating metal surfaces.

. Generally, when a cathode electrolysis is carried out in an aqueoussolution containing hexavalent chromium ion, a treating time of morethan 20 seconds Will be required in order to obtain an expectedelectrodeposited film;

The present invention is a method of chemically treating metals in whichthe above mentioned electrolytic treating time can be remarkably reducedand which is characterized by carrying out a cathode electrolytictreatment in a solution'containing hexavalent chromium ion and a slightamount of halogen ion. In spite of the very short treating time, themetal surface treated according to the present invention has a highanticorrosiveness and paint adhesiveness and shows a beautifulinterference color. There are three elements which constitute theimportant part of the present invention; the hexavalent chromium ion,halogen ion and cathode electrolysis. Therefore, if any one of them islacking, no satisfactory product will be obtainable. The presence of aslight amount of halogen ion suchas chlorine ion or fluorine ion is ofespecially important significance.

When a cathode electrolysis was carried out in a chromic acidelectrolyte containing no halogen ion at all, as described above, anelectrolyzing time of more than 20 seconds was required. It is alsoknown to add halogen ion in a chromic acid treatment, but not in anelectrolytic A ode electrolysis is carried out by using an aqueoussolutionprepared by adding halogen ion'to a solution of a hexavalentchromium compound. When 25, 50 and 100 p.p.m. (parts per million) of ahalogen ion are added, the same amount of deposition will be obtainableby elec- With the increase in the amount of addition of a halogen ionwhich is to be an adjuvant the amount of production of anelectrodeposited film will increase. But, if the amount of addition istoo large, the film will be likely to peel off. Therefore, the range ofaddition of 10 to 200 p.p.m. is preferable' As a means of supplyinghalogen ion, there is added such acid, salt or soluble complex salt asdissociates to yield halogen ion as, for example, hydrochloric acid,bydrofiuoric acid chloride or fluoride, fluotitanate or boron fluoride.

When the cathode electrolysis is continued for a long time in thetreating solution of the present invention, a trivalent chromiumion willcome to be produced in the solution. However, a trivalent chromium ionmay be contained in the treating solution in advance for improving thedryability of the solution.

Phosphoric acid will give a property of improving the paint adhesivenessand the uniform electrodepositability of the electrodeposited film andis therefore a favorable additive. It is necessary to keep the amount ofaddition of phosphoric acid to below 10 g./l. If this amount isexceeded, no adhesive film will be produced.

When the pH of the solution is above- 1.3, the electrodeposited filmwill become very hard to produce. Therefore, the pH should be kept below1.3.

The lower the liquid temperature, the larger the amount of the filmproduced by the electrolysis. With the rise of the liquid, temperaturethe amount of production of the film will gradually decrease. However,the treatment in a range of 10 to 80 C. is feasible.

The anticorrosiveness of the product depends most upon the amount ofdeposition of the film, and the latter is again mostly determined by theamount of addition of the halogen ion, the current density and theelectrolyzing time. The amount of film produced will increase almosttrolysis for 4, 2 and 1 second, respectively. Without the halogen ion,no chemical treatment can be achieved in such a short time. Such higheffect of a halogen of accelerating the formation of a film in a cathodeelectrolytic treatment has been heretofore quite unknown.

The treating method accordingto the present invention shall now beexplained in detail. The present invention is a chemical treating methodwherein a cathode elec trolysis is carried out in a solution in which 5to 50 g./l. of hexavalent chromium ions and 10 to 200 p.p.m. of chlorineor fluorine ions are added, the pH is adjusted to less than 1.3 and theliquid temperature is kept at 10 to 80 C. I

First of all, the composition of the present treating solution shall bedescribed. As supply sources of the hexavalent chromium ion which is tobe a main ingredient of the present treating solution can be considereda chromic anhydride, dic-hromate and chromate. But usually a chromicanhydride is used. In case a film is to be formed by using a dichromateor chromate, it will be necessary to adjust the pH to below 1.3 byadding nitric acid or phosphoric acid.

in proportion to increase in current density and to increase inelectrolyzing time.

Any condition may be selected for the current density and electrolyzingtime but they are in a relation of an inverse proportion. That is tosay, in order to obtain the same amount of deposition, the electrolyzingtime may be shorter, if the current density is higher and vice versa.Thus, by adjusting the amount of addition of the halogen ion, thecurrent density and the electrolyzing time, a film having any amount ofdeposition can be obtained.

The color of the treated product will vary depending on the amount ofdeposition of the electrodeposited film. With the increase in the amountof deposition of the electrodeposited film, the color will change tolight brown (below 10 mg./ft. bluish brown (at 10 to 20 mg./ft. blue (at20 to 30 mg./ft. yellowish blue to yellow (at 20 to 40 mg./ft. red (at40 to 50 mg./ft.) and green (at 50 to mg./ft. in turn. Therefore, thepresent invention can be utilized also as a method'of coloringmetals.

However, practically by carrying out the treatment to such a degree aswill produce blue or yellowish blue (at 10 to 30 mg./ft. suflicientanticorrosiveness and paint adhesiveness may be obtained.

As a drying method after the electrolytic treatment, the

product may be squeezed with rolls and then dried to 1 dry the residualsolution or may be washed with water after the electrolysis to wash offthe solution.

When drying the solution by squeezing, the solution is easier to dry, ifa trivalent chromium ion is present in the solution. Defatting must becarried out as a pretreatment. But, in case the material is not rusted,acidwashing is not necessary. And the anticorrosiveness Will beimproved, if the steel sheet is painted With a hexavalent chromium ionbefore the electrolytic treatment is carried out. As such hexavalentchromium ion solution may be used either the same solution as thetreating solution or a separate chormic anhydride solution.

The nature of the film obtained by the method of the present inventionis not yet clarified enough. However, in the observation by using anelectron microscope and an electron diffraction, apparatus neithercrystalline film nor metallic chromium has been recognized. Thus thefilm is considered to be an a morphous gel. Judging from this result,the mechanism of producing the film by the method of the presentinvention is considered to be that, when a metal is cathode-electrolyzedin a solution containing 21 hexavalent chromium ion, a trivalentchromium ion will be produced on the metal surface as a result of thereduction of hexavalent chromium ion, at the same time the pH will riseon the surface and the trivalent chromium ion will be deposited on thesurface in the form of a gel as chromium chromate.

In what part of the film production the slight amount of the halogen ionhas a catalytic effect is not yet known. However, it is clear that, incase no halogen ion is contained, chromium chromate will be very hard todeposit and that, with the addition of the slight amount of the halogenion, the production of the film will be accelerated very much.

Even when the thus produced film is washed with water, it will not peelofi. When it is dried, it will further stick to the steel sheet and willbecome high in hardless. Further, with the lapse of time, the film willgradually show water repellency and its anticorrosiveness will come toimprove.

Example 1 A steel sheet was electrolyzed and defatted with an aqueoussolution of an alkali, was then washed with water, was dipped at thenormal temperature in a 1%- chromic acid solution and wascathode-electrolytically treated under the following conditions by usingpure wa- .ter:

Chromic anhydride g./l 35 Phosphoric acid g./l 7 Sodium chloride g./l0.05 Temperature of the treating solution C 60 Current density of thecathode a./ft. 200 Electrolyzing time second-s 3 Example 2 Chromicanhydride g./l 5O Hydrochloric acid g./l 0.05 Temperature of thetreating solution C 30 Current density of the cathode a./ft. 150Electrolyzing time seconds 2 Example 3 Chromic anhydride g./l 50 Sodiumfluoride g./l 0.1 Temperature of the treating solution C 50 Currentdensity of the cathode a./ft. 200 Electrolyzing time seconds 3 Example 4Chromic anhydride g./ 1 40 Sodium chloride g./l 0.12 Temperature of thetreating solution C 50 Current density a./ft. 250 Electrolyzing time-second 1 In case the steel sheets treated as mentioned in the aboveExamples 1 to 4 were left to stand in an ordinary room, they were notdamaged by rusting even after the lapse of 6 months. Even when they weretested in a moisture tank at a humidity of 100% and a liquidtemperatu're of 49 C. for one week, they were not seen to rust. And evenin the dew point corrosion test for one week in an outer tank at 49 C.and an inner tank at 39 C., they did not rust. Further, when they Weresubjected to a salt water spray test according to J ISZ 2371 (whereinsalt water was sprayed against a wall through a nozzle at a salt waterconcentration of 5%, a tank temperature of 35 C. and a test piece angleof 15 degrees so that the rising salt water mist might be sprinkled onthe surface of the test piece), even after the lapse of 6 hours, theywere not seen to rust.

In the same salt water spray test, a steel sheet treated with aphosphate and a steel sheet not treated at all usually produced red ruston the whole surfaces in about 30 and 5 minutes, respectively. On theother hand, the' Example 5 A steel sheet was cathode-electrolyzed underthe following conditions, the product then squeezed with rolls and theresidual solution dried:

Chromic anhydride g./l 30 Chromium acetate (as trivalent chromium ion)g./l 2.5 Sodium chloride (as Cl) p.p.m 50 Temperature of the treatingsolution C 50 Current density of the cathode a./ft. Electrolyzing timeseconds" 4 As a drying condition in this case, the steel sheet could bedried in 4 seconds in a drying furnace at 500 C. But, when no trivalentchromium ion was present, even when the steel sheet was dried for 4seconds in a drying furnace at 800 C., it could not be Well dried andshowed a hy-grosco-pi-city.

Example 6 An electrogalvanized steel sheet was cathode-electrolyzedunder the following conditions:

Chromic anhydride g./l 35 Sodium chloride (as Cl) p.p.rn 70 Temperatureof the treating solution C 60 Current density of the cathode a./ft. 100

Electrolyzing time "seconds" 4 Example 7 An electr-ogalvanized steelsheet was cathodeelectrolyzed under the following conditions:

Chromic anhydride g./l 30 Sodium chloride (ascl' p.p.rn 40 Temperatureof the treating solution C 40 Current density of the cathode 'a./ft. 600Electrolyzin g time second 0.5

When the thus treated galvanized steel sheet was subjected to a saltwater spray test for hours, it was not seen to produce any red rust. Butan untreated galvanized steel sheet produced red rust in 2 hours.

Example 8 A molten-aluminum plated steel sheet was cathodeelectrolyzedunder the following conditions:

Chromic anhydride g./l 35 Sodium chloride (as Cl-) p.p.m 50 Temperatureof the treating solution C 40 Current density of the cathode a./ft. 600Electrolyzing time seconds 2 The film produced by this treatment showedno interference color and was colorless.

When an aluminum plated steel sheet not treated in the above mentionedmanner was subjected to a salt water spray test for 120 hours, itproduced white rust on the whole surface. But the sheet treated in theabove mentioned manner produced no white rust at all.

What we claim is:

1. A method of chemically treating a metal article which compriseselectrolytically treating said metal article in an aqueous solution,making said metal article a cathode, said aqueous solution consisting ofa hexavalent chromium compound yielding hexavalent chromium ions in anamount of 5-50 :g./l. as its main ingredient and a substance producinghalogen ions in an amount of 10 to 200 ppm. as adjuvant, the pH of saidsolution being below 1.3 and the temperature thereof being 10 to 80 C.

2. A method according to claim 1 wherein said aqueous solution also haspresent phosphoric acid in an amount of less than 10 g./ 1.

3. A method according to claim 1 wherein said aqueous solution alsocontains trivalent chromium ions.

References Cited by the Examiner UNITED STATES PATENTS 2,733,199 1/1956Wick 20456 2,812,296 11/1957 Neish 204--56 X 2,998,361 8/1961 Kitamura20456 3,032,487 5/1962 Yo-nezaki et a1. 20456 3,118,824 1/ 1964 Yonezakiet al. 20456 3,175,964 3/1965 Watanabe et a1. 20456 X JOHN H. MACK,Primary Examiner.

G. KAPLAN, Assistant Examiner.

1. A METHOD OF CHEMICALLY TREATING A METAL ARTICLE WHICH COMPRISESELECTROLYTICALLY TREATING SAID METAL ARTICLE IN AN AQUEOUS SOLUTION,MAKING SAID METAL ARTICLE A CATHODE, SAID AQUEOUS SOLUTION CONSISTING OFA HEXAVALENT CHROMIUM COMPOUND YIELDING HEXAVALENT CHROMIUM IONS IN ANAMOUNT OF 5-50 G./L. AS ITS MAIN INGREDIENT AND A SUBSTANCE PRODUCINGHALOGEN IONS IN AN AMOUNT OF 10 TO 200 P.P.M. AS ADJUVANT, THE PH OFSAID SOLUTION BEING BELOW 1.3 AND THE TEMPERATURE THEREOF BEING 10 TO80*C.